1. Field of the Invention
The present invention relates to a refrigerating cycle having a plurality of evaporators. More particularly, the present invention is preferably applied to a supercritical refrigerating cycle in which a refrigerant such as CO2 (carbon dioxide), the pressure of which is not less than the critical pressure (the refrigerant is in the supercritical state), is used.
2. Description of the Related Art
Concerning this type of supercritical refrigerating cycle, it is normal to use a supercritical refrigerating cycle, as shown in FIG. 22, in which a plurality of evaporators 5, 11 are connected in parallel with each other. Concerning this prior art, refer to FIG. 1 of the official gazette of JP-A-2000-35250. According to this prior art, the first pressure reducing device 4 for reducing the pressure of a refrigerant, which flows into the first evaporator 5, which is one of the plurality of evaporators 5, 11, is composed of an electrical valve mechanism, and the temperature of the refrigerant at the outlet of the radiator 2, which is located outside, is detected by the temperature sensor 21 and, further, the pressure of the refrigerant at the outlet of the radiator 2 is detected by the pressure sensor 22.
When the degree of opening of the first pressure reducing device 4 is controlled by a control signal outputted from the control unit 20, the pressure of the refrigerant at the outlet of the radiator 2 is controlled to a target pressure which is determined by the temperature of the refrigerant at the outlet of the radiator, so that the operation efficiency (COP) of the refrigerating cycle can be enhanced. The accumulator 9 is arranged on the outlet side of the first evaporator 5, so that the liquid refrigerant can be prevented from being sucked into the compressor 1 through the passage at the first evaporator 5 side.
On the other hand, the second pressure reducing device, which is arranged in parallel with the first pressure reducing device 4, is composed of a temperature-type expansion valve 100. This temperature-type expansion valve 100 reduces the pressure of the refrigerant flowing into the second evaporator 11. The temperature-type expansion valve 100 has a temperature sensing portion 100a, the inner pressure of which is changed according to the temperature of the refrigerant at the outlet of the second evaporator 11, and conduct control of the degree of superheating of the refrigerant at the outlet of the second evaporator 11. Due to the foregoing, liquid refrigerant can be prevented from being sucked into the compressor 1 through the passage at the second evaporator 11 side.
According to another example described in the above Patent Document, and as shown in FIG. 23, the second pressure reducing device is composed of a fixed throttle 10, and the outlet side of the second evaporator 11 is joined to the outlet side (the suction side of the compressor) of the accumulator 9. Concerning this prior art, refer to FIG. 14 in the official gazette of JP-A-2000-35250.
In this connection, according to the prior art shown in FIG. 22, the following problems may be encountered. As the temperature-type expansion valve 100, which independently conducts control of the degree of superheating of the refrigerant at the outlet of the second evaporator 11 in accordance with a change in the refrigerating load given to the second evaporator 11, is used as the second pressure reducing device, when the refrigerating load given to the second evaporator 11 is increased, the behavior shown in FIG. 24 is caused, and the high pressure control cannot be stably conducted by the first pressure reducing device 4 and hunting is caused in the circuit. As a result, the operation efficiency (COP) of the cycle is deteriorated.
According to the prior art shown in FIG. 22, in the case where a volume of air of the second evaporator 11 is suddenly decreased and the refrigerating load is sharply reduced or in the case where a rotating speed of the compressor 1 is suddenly increased and the low pressure is decreased, the following serious problems are caused. That is, when the low pressure is decreased, as the responding property of the temperature sensing portion 100a is much lower than that of the pressure sensing portion in the temperature-type expansion valve 100, the degree of superheating is increased too much.
Due to the foregoing, in the temperature-type expansion valve 100, the degree of the valve opening is increased to the substantially fully opened state. As a result, almost all the refrigerant circulating in the cycle flows onto the second evaporator 11 side, and the liquid refrigerant is returned at the outlet of the second evaporator 11. On the other hand, at the first evaporator 5 side, a shortage in the flow of the refrigerant is caused. Therefore, the degree of superheating of the refrigerant at the outlet of the first evaporator 5 is excessively increased.
Further, at this time, the temperature-type expansion valve 100 is substantially fully opened, and the high pressure is decreased. Accordingly, a shortage in the flow of the refrigerant at the first evaporator 5 side is further facilitated, and the refrigerating performance of the first evaporator 5 is greatly deteriorated.
In the case where the refrigerating cycle is started from the stop state, when the low pressure is decreased according to the starting operation of the compressor 1, the degree of the valve opening of the temperature-type expansion valve 100 is substantially fully increased in the same manner as that described above, and a shortage in the flow of the refrigerant at the first evaporator 5 side is caused. Accordingly, it is impossible to excellently exhibit the refrigerating performance of the first evaporator 5. As the pressure is not quickly increased, at the time of starting, the entire refrigerating cycle does not function well.
On the contrary, when the low pressure is raised because the refrigerating load is suddenly increased or the rotating speed of the compressor is suddenly decreased, the degree of superheating is too low in the temperature-type expansion valve 100, and the degree of the valve opening is sharply reduced. Therefore, a poor refrigerating performance at the second evaporator 11 is caused.
On the other hand, according to the prior art shown in FIG. 23, as the second pressure reducing device is composed a fixed throttle 10, it is possible to solve the problems caused when the temperature-type expansion valve is used. On the other hand, the following problems, which are different from the problems described above, are caused.
The diameter of the hole of the fixed throttle 10 composing the second pressure reducing device must be determined so that a volume of the refrigerant corresponding to the necessary performance at the time of the maximum refrigerating load of the second evaporator 11 can be made to flow in the hole of the fixed throttle 10. However, when the diameter of the hole of the fixed throttle is set at the diameter necessary at the time of the maximum refrigerating load, when the refrigerating load of the second evaporator 11 is light, that is, when the volume of air is small or the temperature of the sucked air is low, a volume of the refrigerant flowing at the second evaporator 11 side is excessively increased, and the refrigerant at the outlet of the second evaporator 11 contains too much liquid refrigerant.
As a result, the liquid refrigerant is returned to the compressor 1, and an excessively high stress is generated by the compression of liquid, which affects the life of the compressor. Further, as the liquid refrigerant cannot be effectively utilized to exhibit the refrigerating performance of the evaporator, the cycle operation efficiency is deteriorated.
On the contrary, when the diameter of the hole of the fixed throttle is set at a value lower than the diameter necessary at the time of the maximum cooling load, the flow rate of the refrigerant flowing in the second evaporator 11 is low when a heavy load is given to the second evaporator 11, and the degree of superheating of the refrigerant at the outlet of the second evaporator 11 is excessively raised. Accordingly, the refrigerating performance is poor. Further, due to this excessively high degree of superheating, a large temperature variation is generated in the air blown out from the second evaporator 11, white fog is generated in the air blown out from the second evaporator 11 and, further, the air conditioning feeling is deteriorated by the large temperature variation in the vehicle passenger compartment.